In fuel injected engines, it is essential to ensure that the pressure of fuel delivered to the injectors is accurately controlled so that each time an injector opens the desired quantity of fuel is forced through the injector nozzles into the cylinders of the engine.
In order to ensure that the correct fuel pressure is obtained, it is conventional to supply the injectors with fuel from a fuel rail, and for the fuel rail to be connected to the fuel tank through feed and return passages. The fuel pump operates continuously, at a constant rate and pumps more fuel per unit time to the rail than is required. The excess fuel is returned to the tank through the return line. A pressure regulator is conventionally incorporated in or near the fuel rail.
Proposals have been made to operate fuel injection systems with only one fuel pipe leading between the fuel tank and the fuel injectors at the engine. Such systems are conventionally known as `returnless` fuel systems. The fuel pump in such a system will operate in accordance with engine demand and under certain conditions (e.g., deceleration) will stop running and the injectors will be closed. It is important then that the correct fuel system operating pressure is maintained at the injectors so that when demand returns and the injectors are operated, fuel at the correct pressure is present at the injectors.
In order to maintain this pressure, a check valve can be provided in the fuel line, as described in U.S. Pat. No. 4,964,391. However the volume of fuel trapped between the check valve and the injectors can be subject to external influences, for example temperature changes, which will alter the pressure under which the trapped volume is held. Such changes are undesirable.